Book Image

Linux Networking Cookbook

By : Agnello Dsouza, Gregory Boyce
5 (1)
Book Image

Linux Networking Cookbook

5 (1)
By: Agnello Dsouza, Gregory Boyce

Overview of this book

Linux can be configured as a networked workstation, a DNS server, a mail server, a firewall, a gateway router, and many other things. These are all part of administration tasks, hence network administration is one of the main tasks of Linux system administration. By knowing how to configure system network interfaces in a reliable and optimal manner, Linux administrators can deploy and configure several network services including file, web, mail, and servers while working in large enterprise environments. Starting with a simple Linux router that passes traffic between two private networks, you will see how to enable NAT on the router in order to allow Internet access from the network, and will also enable DHCP on the network to ease configuration of client systems. You will then move on to configuring your own DNS server on your local network using bind9 and tying it into your DHCP server to allow automatic configuration of local hostnames. You will then future enable your network by setting up IPv6 via tunnel providers. Moving on, we’ll configure Samba to centralize authentication for your network services; we will also configure Linux client to leverage it for authentication, and set up a RADIUS server that uses the directory server for authentication. Toward the end, you will have a network with a number of services running on it, and will implement monitoring in order to detect problems as they occur.
Table of Contents (19 chapters)
Linux Networking Cookbook
Credits
About the Author
About the Reviewer
www.PacktPub.com
Preface
Index

Scanning UDP ports


It is very easy to read the results of a TCP scan due to its stateful nature. A SYN packet will always be answered with a FIN if the port is closed or a SYN/ACK if the port is opened. The lack of a response means that either the request or its response was filtered.

UDP is not so easy, due to it being stateless. A UDP packet to a closed port will result in an ICMP Destination Port Unreachable message. A filtered UDP packet will result in no response. The tricky part is that the behavior when something is listening to the port is application specific. Since there is no initial handshake, the application simply receives the data and then either responds or not depending on the application's requirements. If the application does not respond, it will look just like a filtered port.

How to do it…

Similar to SYN scans, UDP scans require root privileges. Simply use –sU in order to specify UDP for the scan type.

Before we run the scan, let's add UDP filtering on port 22 in order to...